The property that a finite-difference approximation is formulated in such a way that it has access to the information that is required to determine the solution of the corresponding differential equation; violating this condition leads to a numerical instability. As an example, suppose the solution to the differential equation is a wave traveling at speed c. If a finite-difference approximation is only able to access information on its grid that is traveling at speeds less than c, it violates the CFL condition and it will not be able to approximate the solution of the differential equation.